Hydrogen sulfide has long been known to be objectionably odorous even in very low concentrations in air, as well as extremely toxic. Its odor is objectionable to most people in concentrations as low as a few parts per million by volume, and is detectable and even recognizable in concentrations as low as a few parts per billion. G. Leonardos, J. Air Pollution Control Association, Vol. 19, p. 91 (1969), has reported the odor threshold for hydrogen sulfide to be 0.47 parts per billion by volume in air. Furthermore, hydrogen sulfide is produced by many natural as well as industrial processes or operations. For example, the bacterial decomposition of sulfur-containing organic matter produces hydrogen sulfide, and therefore this gas is present in sewage, garbage dumps, sulfur springs, and rotting vegetation that is rich in sulfur, such as cabbages, broccoli, and brussels sprouts. In addition, the production of pulp and paper, the purification of waste water, and the storage and transfer of petroleum products release hydrogen sulfide into the ambient atmosphere. For these reasons considerable effort has been made toward developing improved methods for the removal of hydrogen sulfide from air. U.S. Pat. No. 4,072,479 describes the use of a bed of activated carbon treated with sodium hydroxide and moisture for removing hydrogen sulfide and other sulfur compounds. The saturated or exhausted carbon can be regenerated by treatment with dilute sodium hydroxide solution.
Some disclosures have been made regarding the use of ammonia gas as an aid to the removal of hydrogen sulfide from air. U.S. Pat. No. 2,168,933 teaches that hydrogen sulfide can be removed from technical gases such as water gas by the use of activated carbon with the addition of small amounts of air and ammonia. British Pat. No. 337,348 also discloses the use of ammonia, but this is for the reactivation of carbon, not for its use as an adsorbent. Engelhardt (Zeitschrift fur angewandte Chemie, Vol. 34, pp. 293-296, (1921)), describes the addition of ammonia to a gas stream containing hydrogen sulfide and oxygen for recovery of the sulfur. The same process is referred to in U.S. Pat. No. 4,072,479.
In our studies of the removal of hydrogen sulfide from air containing the same, we have confirmed that the addition of ammonia serves to enhance the removal of the hydrogen sulfide from air when an air stream containing both the hydrogen sulfide and ammonia is passed through activated carbon. However, the introduction of ammonia into the hydrogen sulfide-containing air stream frequently resulted in the formation of and in the deposition of solid ammonium polysulfide in the equipment through which the air stream is passed. The build-up of these deposits eventually required the dismantling of the equipment to remove these deposits since they diminished the rate of flow of the air stream to the activated carbon thus slowing the rate of the process. Such dismantling operations are time consuming and costly.